Field of the Invention
The present invention relates to a liquid crystal display device in an IPS (In-Plane Switching) mode or an FFS (Fringe Field Switching) mode.
Description of the Background Art
Electrodes of a liquid crystal display device in the IPS mode are arranged such that an electric field is applied in parallel to the liquid crystal surface. The IPS panel has an active matrix driving electrode including a pair of electrodes that are arranged in the same plane. The IPS panel can rotate a direction of liquid crystal molecules in the panel surface. Therefore, the IPS panel has an excellent view angle characteristic.
The liquid crystal display device in the FFS mode is a display system that performs display by adding a fringe electric field to the liquid crystal which is sandwiched between counter substrates. According to the liquid crystal display device in the FFS mode, the pixel electrode and the counter electrodes are formed of transparent conductive films, so that an opening ratio and transmittance that are higher than those in the IPS mode can be obtained.
It is general that an orientation film is formed on top surfaces of two substrates on the liquid crystal layer side and orientation of liquid crystals is controlled by a rubbing process of the orientation film. The orientation film has a pretilt angle in the range between 0 degrees or above and 5 degrees or below. Due to the pretilt angle, a distortion occurs in the view angle characteristic.
In U.S. Pat. No. 6,285,430, for example, there is disclosed a liquid crystal display device in the IPS mode which uses a positive biaxial retardation film (also referred to as a biaxial retardation film). By using the positive biaxial retardation film, black level luminance of the liquid crystal display device in the IPS mode can be reduced in the azimuth angle direction (an oblique direction) of particularly 45 degrees, 135 degrees, 225 degrees, and 315 degrees.
Further, in Japanese Patent Application Laid-Open No. 2014-115563, for example, there is disclosed in detail a configuration of a liquid crystal display device in the IPS mode which uses a positive biaxial retardation film. That is, the liquid crystal display device includes a liquid crystal layer sandwiched between a first substrate and a second substrate, a biaxial retardation film arranged on the side where light enters the liquid crystal layer, a first polarization plate arranged on the outer side of the biaxial retardation film, and a second polarization plate provided on the opposite side of the first polarization plate by sandwiching the liquid crystal layer. An orientation direction of the liquid crystal layer is set in a direction parallel to the extending direction of a common wire. A slow axis of the biaxial retardation film is set in a direction parallel to or orthogonal to the orientation direction of the liquid crystal layer. One of absorption axes of the first and second polarization plates is set in a direction parallel to the orientation direction of the liquid crystal layer, and the other is set in a direction orthogonal to the orientation direction. As described above, according to a configuration in which a biaxial retardation film is arranged on the light incident side below the liquid crystal layer, it is possible to obtain a desired optical compensation effect without receiving the influence of a disordered state of the liquid crystal layer.
Further, in Japanese Patent Application Laid-Open No.2009-276485, for example, there is disclosed a structure in which in order to suppress the increase in black level luminance due to polarization cancellation of reflected light on the side surface of a metal wire, a direction of the side of the metal wire is set orthogonal to or parallel to a direction of the polarization.
Further, in Yuka Utsumi, Shintaro Takeda, Hiroyuki Kagawa, Daisuke Kajita, Bum Hiyama, and Yasushi Tomioka, &#34; Improved Contrast Ratio in IPS-Pro LCD TV by Using Quantitative Analysis of Depolarized Light Leakage from Component Materials&#34;, SID 08 DIGEST, 2008, p.129, for example, it is described that existence of scattering leakage light due to a color filter (CF) and a liquid crystal molecules gives an non-negligible influence to the front contrast ratio (front CR).
In a liquid crystal display for an on-vehicle center console, it is desirable to realize high contrast in an angle area in the front direction surrounded by an upper left oblique direction and an upper right oblique direction (polar angle 42 degrees and azimuth angle 24 degrees, and polar angle 42 degrees and azimuth angle 156 degrees) and a lower left oblique direction and a lower right oblique direction (polar angle 42 degrees and azimuth angle 12 degrees, and polar angle 42 degrees and azimuth angle 178 degrees). However, the liquid crystal display has a view angle characteristic that contrast is unbalanced in a specific direction, such as the contrast in the upper right oblique direction (polar angle 42 degrees and azimuth angle 24 degrees) is high but the contrast in the left direction (polar angle 42 degrees and azimuth angle 156 degrees) is low.
To cope with such a problem, in the liquid crystal display device in the IPS and FFS modes, by using the biaxial retardation film, black level luminance not only in the front direction and left and right directions but also oblique direction has been reduced to some extent.
In a conventional liquid crystal display device, a proper view angle characteristic is designed, by performing a calculation considering the incident angle (polar angle and azimuth angle) of coherent parallel light, axis angle of the polarization plate, birefringence and orientation state of liquid crystal molecules, axis angle and birefringence index of the phase difference plate, by using a general-purpose simulator (an LCD master, for example) that performs a polarization calculation using a Jones matrix.
However, in the liquid crystal display device in the FFS mode that uses the biaxial retardation film disclosed in Japanese Patent Application Laid-Open No. 2014-115563, divergence between the calculation value and the measurement value tends to become large. Further, there is still a problem in that black level luminance in a specific direction is high and the view angle characteristic has a distorted distribution.
In the light of the content of Japanese Patent Application Laid-Open No.2009-276485, this problem is considered because the influence of scattering light that is not considered in the polarization calculation using the Jones matrix relatively increases. Because scattering leakage light is radiated to a direction different from a direction of an incident light ray, the inventor has considered that the oblique light of the backlight (BL) also has the influence to the front CR. That is, the inventor has considered that the existence of scattering leakage light in a specific view angle direction gives a non-negligible influence to black level luminance A prior-art literature concerning a quantitative influence of scattering leakage light to black level luminance and a change thereof due to the configuration of a liquid crystal display device has not been disclosed so far as the inventor knows.